A conventional laser CVD device of this type is as shown in FIGS. 2 and 3. In the device, an ultraviolet laser 1, such as an excimer laser is employed as a light source, and its laser beam 2 is directed into a reaction chamber 4 through a window 3. A substrate 6 is mounted on a susceptor 5 in the reaction chamber 4, and is heated to a required temperature by a heater 7. In forming a silicon film, a gas mixture prepared by mixing a base gas, namely, silane gas (SiH.sub.4 or Si.sub.z H.sub.6) with a diluting gas (rare gas or H.sub.z) is employed as a raw gas when forming a nitride film and an oxide film. When forming a nitrogen film and an oxygen film, the gas mixture is prepared by mixing the base gas with nitrogen gas and oxygen gas as raw gases.
The raw gas is introduced into the reaction chamber 4 which has been evacuated through the discharge outlet 9. The raw gas thus introduced reacts opto-chemically with the laser beam 2, to form the radical. The radical accumulates on the substrate 6 to form a thin film. In this operation, the laser beam may be applied directly to the substrate as shown in FIG. 2, or it may indirectly illuminate the substrate as shown in FIG. 3.
As is apparent from the above description, in the conventional device, the laser beam passes through only a part of the space in the reaction chamber, and therefore the efficiency of formation of the radical is low. Furthermore, in the case where a plurality of raw gases are provided, it is rather difficult to use them selectively. That is, in forming an alloy film by using a plurality of raw gases, it is difficult to control the composition ratio of them over a wide range.